In the manufacture of printed circuit boards it has been necessary to connect coils for inductors and transformers to the other board circuitry by physical attachment of the coils to the boards. In present practice the coils have ferrite cores, and, where tuning is required, the cores have tunable ferrite slugs. As the final step in the physical manufacture of a printed circuit board it is customary to apply a lacquer-like protective layer, known as a conformal coating, over the entire surface of the board. This is done by dipping the completed board in an acrylic solution. On a production basis printed circuit boards are carried on a conveyor through a tank of the solution and through an air drying station, following which they are stored for testing and final tuning of any tunable inductors. It has been found, however, that the final tuning is obstructed, in many instances, by the conformal coating which fills the fine threads of the tuning device and/or binds the slug in its off-tune position.
Although the conformal coating operation does apply a protective layer to the wire of the induction coil, it does not encapsulate the coil to a degree that moisture is excluded from the turns, and changes in moisture content can cause variations in the inductive reactance of inductors on conventional printed circuit boards. Sealing of the core containing the wire coil by means of silicone rubber has been practiced but this expedient has the disadvantages of high cost and the possible entrapment of any moisture that may find its way into the coil area.